People ought to know what you're talking about. If you said: Hier is een briefje voor jou, and handed someone a small piece of paper, full with writing, they would see a small piece of paper with a lot of information written on it, and think the PIECE OF PAPER IS SMALL, but that it's just a very detailed note. But typically, a "briefje" refers to a SMALL AMOUNT OF INFORMATION, regardless of the size of the paper.
If you think briefje = letter, and type it and it's wrong, based on my explanation above, you should report it.
Point taken; but many of the other diminutives are accepted with or without the "little" as understood. vraagjes (Little questions) but "questions" alone is always accepted. You could say a kopje is a demitasse, or "little cup"... but "cup" is always accepted. Not about to argue with one who clearly knows the language; but just saying that this seems to be an inconsistency. I definitely understand that a briefje is a "little" letter... perhaps what we need to take from this sentence is that some diminutives (like this one) will always translate into a different word. In the case of this one... it must be a note.